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If the aliens in the movie Signs could be poisoned by

water,

why didn't the atmosphere kill them? Why would it have to be in liquid form?

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    I got the impression that water burned them (much like acid would), not that it poisoned them. – onewho Aug 23 '12 at 21:51
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    Oh, don't even get me started on this plot hole. ;) – gnovice Aug 23 '12 at 21:51
  • I can't compete with @Thaddeus's answer, but it was observed by one of the characters that none of the ships/signs occurred near water. Living near the Mississippi, I can verify that near the water, there is more moisture in the air. (yeah, I know, duh, common knowledge...) I got the impression that the aliens chose to attack further from water for this very reason. Likely moisture in the air did affect them, but they were able to tolerate it better where there was less of it. – David Stratton Aug 24 '12 at 4:35
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    Why would aliens decide to colonize a planet that is mostly water? Also they seemingly walked around naked as if there is no dew. It is indeed a ridiculous plot hole – user57248 Dec 15 '15 at 14:57
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    I haven't seen the movie, but isn't this title a major spoiler? – ThePopMachine Dec 15 '15 at 15:38
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The aliens in this movie were not poisoned, per se by water. They instead acted as if they had been burned by a strong acid. Water vapor may have a similar effect but only if they are exposed to it long term. Depending on the water density in the atmosphere, it may act mostly as an irritant in the short term.

Water COULD function as an acid, but it requires a number of conditions to be met. It is a rather simplistic idea that, because the pH of water is 7, it is "neutral." It isn't.

  • Water has 2 lone pairs of electrons on the oxygen, one of which can be donated to a suitable acceptor. Water is therefore a Lewis base.

  • Water also contains hydrogens, which under suitable conditions can be donated as protons. Therefore, water is a potential Bronsted-Lowry acid.

  • In many environments, it will act as neither. But in the right environment, with suitable reagents, it can act as either

Explaining this without invoking acidity/basicity:

  • 2H2O (l) <-----> H3O+(aq) + OH-(aq)

In the presence of strong hydrides, water acts as an acid:

  • NH2- + H2O <-----> NH3 + OH-

In the presence of strong acids, water acts as a base:

  • HCl + H2O <-----> Cl- + H3O+

Water can act as either an acid or a base, dependent on the circumstances.

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    Just a point of fact, you say water is a Lewis base but I believe it is also a Lewis acid... – NominSim Aug 23 '12 at 22:21
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    (Aliens get -1 for attacking a species that's 90% lethal acid, on a planet covered by over 70% lethal acid, where lethal acid randomly falls from the sky, and there's been repeated fiction about how stupid a water-vulnerable alien would have to be to come here. You Fail Tactics Forever.) – Tynam Aug 23 '12 at 22:53
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    There is a quote in the movie where a news caster makes a statement that China had found a primitive way to fight the Aliens... I always pictured a bunch of guys running around with squirt-guns... – onewho Aug 24 '12 at 0:20
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    It's also possible that they aren't actually burned by acid but rather soluble in it. If I made a little igloo out of sugar cubes, it wouldn't spontaneously melt because of humidity, but it would if I poured a glass of water on it. – Paul Jun 15 '14 at 11:06
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    Hard to see microbes through a telescope. – Oldcat Dec 15 '15 at 18:32
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The aliens were killed by belief

The movie isn't an alien invasion movie; it's a movie about faith and redemption. It's the story of the lost and angry ex-Father Graham Hess' road to forgiveness and regaining the strength to be a father, brother and preacher and, most importantly, to accepting his wife's death and forgiving her killer.

The aliens are an allegory for Graham's demons and they may not even be objectively real.

It isn't that water harms them, it's Graham's belief that the water harms them that harms them.

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  • There was also a comment on the radio just before the final "swing Meryl, swing" that the vulnerability to water was "first discovered somewhere in the Middle East, in the early hours of the morning." To me, this has always suggested that God transformed the water into a harmful substance to help repel the aliens ("Holy water" if you will.). I'm no Bible-thumper or anything, but I agree that you can't just ignore the religious aspects of the story. This movie isn't just science fiction. – Steve-O Sep 17 '20 at 14:16
  • Well Merrill saw them, alone independently, without Graham, so they for sure were real. – Rob Jackson Sep 20 '20 at 23:25
  • I don't think he had a grudge per say against Ray. I think he knew Ray didn't kill her on purpose. His grudge was against God for allowing it to happen. – Rob Jackson Sep 20 '20 at 23:26
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It may not have been water. It may have been something in the water. Fluoride? Lead? Any number of other particulates or minerals common to well water but almost completed absent in both oceanic water and within human bodies. They weren't colonizing earth, they were dragging humans away.

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    Welcome to Science Fiction & Fantasy! Do you have any evidence for this theory that you could edit into your answer? We don't mind speculation but prefer answers to be sourced in evidence here, see How to Answer. Also don't forget to register your account and take the tour if you haven't already! – TheLethalCarrot Aug 7 '18 at 16:06
  • There's moisture and water vapor in the air. If the water was killed them, wouldn't they have been dead already? Wouldn't there have been many deaths and fatalities. That's why the water thing doesn't add up. It seemed that something in Bo's water was the killer. The alien invasion wasn't won by the humans easily. It was one long brutal night. – user114025 Aug 19 '19 at 2:48
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I like to think of it like radiation to humans. In small doses (IE everywhere), it doesn't kill you. But concentrated it destroys you.

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  • Can you provide anything that suggests this is true? Constant radiation does indeed kill you? – Edlothiad Jun 15 '17 at 20:46
  • @Edlothiad You are exposed to EM radiation every moment of every day. We have a very good internal DNA repair system – user001 Jun 16 '17 at 10:00
  • I know, but that's tiny amounts, be exposed to slightly more every moment of every day and dead. it doesn't have to be "Concentrated" just more intense. – Edlothiad Jun 16 '17 at 10:03

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